1. Field of the Invention
The present invention relates to a cleaning process used in a semiconductor lithographic manufacturing system and, in particular, to a method for etching or removing an organic hardmask such as amorphous carbon from a low dielectric constant film.
2. Description of Related Art
Integrated circuits (ICs) are fabricated on semiconductor wafer substrates by a lithographic process. The lithographic process allows for a mask pattern of the desired circuit or portion thereof to be transferred via radiant energy of selected wavelengths to a photoresist film on a substrate. Those segments of the absorbed aerial image, whose energy exceeds a threshold energy of chemical bonds in the photoactive component of the photoresist material, create a latent image in the resist. The latent image marks the volume of resist material that either is removed during the development process (in the case of positive photoresist) or remains after development (in the case of negative photoresist) to create a three-dimensional pattern in the resist film. In subsequent processing, the resulting resist film pattern is used as an etch mask to remove underlying substrates from the areas of the patterned openings in the resist layer.
Damascene processing techniques are often used in integrated circuit manufacturing, and involve forming inlaid metal conductors in trenches and vias in a dielectric layer. Openings in a hardmask layer are used to etch the desired portions of the dielectric layer to form the trenches and vias. The hardmask layer openings are themselves formed by etching through openings formed in an overlying resist layer. Hardmasks in damascene process are often made from amorphous carbon, also known as a-carbon or alpha-carbon.
The move from 248 nm to 193 nm wavelength lithography has increased the complexity of the masking integration, often requiring a multi layer stack to be deposited on top of the layer to be etched. An example of this is a three layer stack of an amorphous carbon hardmask layer covered by a SiON anti-reflective coating (ARC) layer on which conventional resist can be spun and processed. After the resist is developed, a fluorine dry etch transfers the pattern to the SiON layer. The resist is stripped in conjunction with an oxygen based etch process to remove a-carbon in the hardmask layer from the openings in the SiON layer. A dielectric etch process then transfers the pattern from the a-carbon hardmask into an underlying dielectric layer used in a dual damascene approach. Following the etching of the dielectric layer, the a-carbon hardmask layer must be removed prior to forming Cu or other metal interconnects in the backend of the wafer process flow.
Sudijono et al. U.S. Pat. No. 6,787,452 discloses a method of controlling a critical dimension during a photoresist patterning process which can be applied to forming vias and trenches in a dual damascene structure. An amorphous carbon ARC is deposited on a substrate by a plasma enhanced chemical vapor deposition (PECVD) method. The alpha-carbon layer provides a high etch selectivity relative to oxide and is disclosed as being readily removed by a plasma ashing step using oxygen. Ye et al., U.S. Pat. No. 6,458,516 teaches a method of removing a polymeric, organic masking layer using a hydrogen/nitrogen-based plasma.
Low dielectric constant (low-k) materials, i.e., those having a dielectric constant generally below about 2.7 to 3.0, have been used in damascene processes as inter-metal and/or inter-layer dielectrics between conductive interconnects employed to reduce the delay in signal propagation due to capacitive effects. The lower the dielectric constant of the dielectric material, the lower the capacitance of the dielectric and the lower the RC delay of the integrated circuit. Typically, low-k dielectrics are silicon-oxide based materials with some amount of incorporated carbon, commonly referred to as carbon doped oxide (CDO). An example of a CDO is CORAL brand carbon-doped oxides, from Novellus Systems, Inc. of San Jose, Calif. It has been found that highly oxidizing conditions are generally unsuitable for use on low-k materials. When exposed to an O2 plasma, the oxygen scavenges or removes carbon from the low-k materials. In many of these materials such as CDOs, the presence of carbon is instrumental in providing a low dielectric constant. Hence, to the extent that the oxygen removes carbon from these materials, it effectively increases the dielectric constant. As processes used to fabricate integrated circuits move toward smaller and smaller dimensions and requires the use of dielectric materials having lower and lower dielectric constants, it has been found that the conventional strip plasma conditions are not suitable.
Consequently, a need exists in the art for the development of an alternative process that effectively removes organic hardmask layers such as amorphous carbon, and that does not remove excessive amounts of the low-k dielectric materials or otherwise materially alter the properties of low-k dielectric materials.